Bruton's tyrosine kinase (Btk) is the target for mutations that cause X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mouse. Both mouse and man have reduced circulating B cells and display defective B cell survival and tolerance. These defects have been primarily attributed to Btk function in B cell antigen receptor (BCR) signaling. Preliminary data presented in this exploratory grant application demonstrate that Btk couples B-cell activation factor receptor (BAFF-R), a TNF-R family member, to the NF- kB pathway and facilitates B cell survival in response to its ligand, BAFF. Emerging evidence suggests that BAFF-R is as critical in B cell survival as BCR. As such, this proposal focuses on defining the molecular mechanisms by which Btk mediates BAFF-R signaling to NF-kB and peripheral B cell survival. Preliminary data further suggests that (i) Btk interfaces with BAFF-R via mechanisms involving TRAF2 and cIAP2, and (ii) Btk kinase function is necessary for BAFF- R-mediated activation of the classical pathway in DT40 B cell system. Based on these and the applicant's published data, a model is proposed in which Btk is linked to BAFF-R via cellular inhibitor of apoptosis protein 2 (cIAP2) and TNF receptor-associated factor 2 (TRAF2) complex and BAFF-R-induced NF-kB pathway involves Btk kinase activity. To test this model, experiments will be conducted to investigate the molecular mechanisms that link BAFF-R to Btk at the level of cIAP2 and TRAF2 and modifications to TRAF3 and NF-kB inducing kinase (NIK) (Aim 1). To determine the biological consequence of Btk and its kinase domain in response to BAFF, Btk-deficient mice will be reconstituted with bone marrow cells expressing kinase-dead Btk and splenic immature B cells will be analyzed for BAFF-R signaling to NF-kB and BAFF- dependent survival. The significance of BAFF-R/Btk pathway in mature B cells will be assessed using mice with floxed Btk gene deletion (Aim 2). The results from the proposed studies will not only significantly impact our mechanistic understanding of a novel function of Btk in BAFF-R signaling, they will also advance our understanding of the mechanisms of B cell survival central to tolerance, autoimmunity and B cell immunodeficiency. PUBLIC HEALTH RELEVANCE: Mutations in the gene encoding Btk result in the B cell deficiency disorders X-linked agammaglobulinemia (XLA) in man and X-linked immunodeficiency (Xid) in mouse. Studies in this grant application will define a novel function for Btk in signaling through B cell activating factor receptor (BAFF-R). The proposed research will illuminate novel mechanisms by which Btk regulates B cell survival critical for normal B cell survival in the healthy immune system as well as B cell autoimmune and immunodeficiency diseases.